Multi-function industrial equipment is known in the art. Many manufacturers produce equipment containing configurable circuits, such as high-voltage and/or high-current windings, that can be selectively configured to provide different functionality. For example, a device including a gas or diesel engine may be selectively configured to operate as a 1000 A welder or as an AC or DC generator. The switches that configure the circuits to perform the selected function are capable of carrying high current, but are not capable of circuit interruption under load. Such interruption may cause arcing, destroying the switch contacts and possibly presenting a fire or shock hazard.
The equipment operator is responsible for understanding the equipment, and knowing what configuration changes can be selected under load. The operator is responsible for reducing or removing the load before changing the equipment configuration, where the configuration change may present a hazard. This reliance on the discretion of skilled and knowledgeable operators routinely results in equipment damage.
In order to remove current from field coils prior to actuation of a switch that would cause current interruption at the switch contacts, equipment manufacturers attempt to detect the onset of switch actuation, and use fast switching circuits such as transistors to change the equipment circuit configuration. A typical rotary switch employing mechanical detents for positive actuation may change state in 20-50 msec. A protection circuit can divert current from a field winding in 10-15 msec, if the onset of switch actuation can be accurately determined.
Prior art switch mechanisms for multi-function industrial equipment utilize mechanical sensors and a precisely positioned cam surface to attempt to detect the onset of actuation of the switch. However, the cam and sensors require precise positioning and careful adjustment to achieve proper detection, increasing the cost of the switch mechanism. Additionally, shock and vibration due to the industrial environment, as well as wear, degrade the reliability of the mechanical sensors.
One known approach to detecting the onset of switch actuation is to incorporate a strain gauge into the switch mechanism, to detect and quantify the force applied to the switch handle. If the force exceeds a threshold, preventive measures such as reconfiguring circuit components can be initiated. However, a strain gauge is a transducer, and it has a mechanical component that must be precisely aligned and configured in the switch mechanism, and is subject to vibration and wear. Additionally, the strain gauge must be properly calibrated. A strain gauge approach is thus deficient for reliable switch actuation onset detection and warning, for the same reasons as mechanical sensors.